Device and process for dissolving smelt from recovery boilers

ABSTRACT

The invention relates to a device for dissolving smelt chemicals from recovery boilers, a so-called soda smelt dissolver, and to a process for such dissolution using the device. The device is characterized in that it consists of at least two separate dissolving tanks (I, II), which are connected to each other near the bottom, via a connecting pipe (3), to allow free flow of liquid between them, and of devices for the separate supply to each tank of dissolving liquid and smelt chemicals in dependence on the measured and established chemical concentration in the solution in each of the tanks (I, II), wherein the outlets from the tanks (I, II) can be separately shut off and regulated. The process according to the invention involves using the device and switching over the functions of the different tanks.

TECHNICAL FIELD

The present invention relates to a device for dissolving smelt chemicalsfrom recovery boilers, a so-called soda smelt dissolver, and also to aprocess for dissolving such chemicals using the device according to theinvention, with the principal advantage of the device and the processbeing that plugging and coating of the device are prevented as a resultof the deposition of pirssonite being decreased.

STATE OF THE ART

When concentrated spent liquor, so-called black liquor, from celluloseproduction is combusted in recovery boilers, smelt chemicals areobtained which are tapped off from the recovery boiler and down into aso-called soda smelt dissolver. This soda smelt dissolver consists of atank which is preferably elongate, and which is equipped with devicesfor supplying smelt chemicals and liquid for dissolving the chemicals,which liquid is normally in the form of so-called weak liquor, which isa dilute aqueous alkaline solution which comes from the lime treatment.Normally, the temperature of the smelt is in the region of 800° C. andthat of the weak liquor is 70°-80° C. The temperature of the solution inthe soda smelt dissolver is approximately 95° C. The soda smeltdissolver is provided with stirring devices and with spraying devicesfor adding the smelt so that the latter, which is at a high temperature,does not produce too violent a reaction when it meets the surface of theliquid, which is at a substantially lower temperature. For tapping it,the tank is provided with two outlet pipes which are placed at adistance from each other.

However, one of the pipes can alternately be used as an inlet pipe forweak liquor while the other is used as an outlet pipe for the solution,so-called green liquor.

TECHNICAL PROBLEM

A continually recurring problem associated with soda smelt dissolvers isthat pipes and valves have a tendency to become plugged with solidmatter and that the stirrers also become coated with such matter. Thissolid matter principally consists of pirssonite, which is a chemicalcombination of sodium carbonate and calcium carbonate. While the sodiumcarbonate derives in the main from the chemical smelt, the calciumcarbonate arrives in the soda smelt dissolver together with the weakliquor. Deposition of this double salt depends partly on theconcentration and partly on the temperature of the solution. The higherthe concentration of alkali and the lower the temperature of thesolution, the more pirssonite is deposited. This problem is described,for example, by W. J. Frederick Jr. and Rajeev Krishnan and also RussellJ. Ayers, in the article Pirssonite Deposits in Green Liquor Processingin the journal Chemical Recovery 1989, pp. 151-156. The need todecrease, or completely eliminate, this deposition problem in soda smeltdissolvers has therefore existed for a long time.

SOLUTION

In accordance with the present invention, therefore, the abovementionedproblem has been solved and a device for dissolving smelt chemicals fromrecovery boilers, a so-called soda smelt dissolver, has been produced,which device includes devices for supplying smelt chemicals anddissolving liquid and also outlets for the resulting solution, whichdevice is characterized in that it consists of at least two separatedissolving tanks, which are connected to each other near the bottom toallow free flow of liquid between them, and of devices for the separatesupply to each tank of dissolving liquid and smelt chemicals independence on the measured and established chemical concentration in thesolution in each of the tanks, wherein the outlets from the tanks can beseparately shut off and regulated.

According to the invention, the device for supplying dissolving liquidincludes a separate conduit from which branches pass to the differenttanks by way of an adjustment valve.

According to the invention, it is expedient for each tank to be providedwith an outlet pump which can be regulated separately.

According to the invention, the devices for measuring the concentrationand the level in the tanks comprise so-called bubble pipes.

The invention also includes a process for dissolving smelt chemicalsfrom recovery boilers using the device according to the invention, andthe process is characterized in that the set point for the concentrationof chemicals in the solution is adjusted to different levels in theseparate tanks and in that the outflow from the tanks, which are in flowconnection with each other, takes place only from the tank which has thehighest set point.

According to the invention, the set points in the different tanks can beswitched alternately, expediently once every twenty-four hours.

According to the invention, one of the set points is expedientlyadjusted to be twice as high as the other.

According to the invention, it can be expedient to arrange for thesupply of the smelt chemicals to the separate tanks to be approximatelyequal, with the majority of the dissolving liquid being supplied to thetank which has the lowest set point.

According to the invention, the procedure on changing the set points andaltering the direction of flow in the connecting pipe between the tanksis first to raise the lower set point to the higher level, after which,when the concentration in the solution has reached this set point as theresult of decreased supply of dissolving liquid to this tank, the outletdevice of this tank is activated and that of the other tank isinactivated at the same time as the set point for this latter tank islowered and the influx of dissolving liquid to this tank is therebyincreased.

According to the invention, the process and the alternation between thetanks can take place automatically.

DESCRIPTION OF THE FIGURES

The invention will be described in more detail below with reference tothe attached figures in which

FIG. 1 is a diagrammatic representation of a vertical section through atank system according to the invention, and in which

FIG. 2 shows the tank system as seen from above.

DETAILED DESCRIPTION

FIG. 1 is a diagrammatic representation of a section through the sodasmelt dissolver according to the invention, which consists of the tanksI and II which, at their lower parts, are connected by means of a pipe3, which pipe 3 enables free flow to take place between tanks I and II.Both these tanks I and II are provided with stirrers 4. In addition,they possess devices 5 for the supply of smelt chemicals from a recoveryboiler. The devices 5 for supplying the smelt are conventional per seand are arranged so that the smelt, which is at a temperature of800°-900° C., is broken up in a spray shower by means of steam beingsprayed against it. This is done in order to dampen the noise andpowerful reactions which otherwise occur when the hot smelt meets thesurface of the liquid, the temperature of which is less than 100° C.Devices 6 for supplying dissolving liquid, preferably in the form ofso-called weak liquor, are also arranged in each tank I and II. The weakliquor, which comes from the calcination, which is included in everysystem for recovering chemicals in recovery boilers, contains a certainamount of calcium which, together with the smelt chemicals, whichprincipally consist of sodium carbonate, sodium sulphide and a smallquantity of sodium sulphate, can result in pirssonite being deposited.According to the invention, the supply of weak liquor through pipes 6 isto be alternated from tank to tank and, for this reason, a common pipe 7has been arranged for this supply, which pipe 7 opens out in a valve 8from which the lines 6 branch off. While the temperature of the weakliquor is normally 70°-80° C., it can be cooled down to a temperature ofapproximately 50°-60° C. As a consequence, problems of reaction or heatevolution do not arise when this weak liquor meets the surface of thesolution.

In addition, both the tanks are provided with outlet openings 9 whichexpediently include a pump 10. The liquid which is withdrawn from one ofthe tanks I or II, and which contains the dissolved chemicals, is termedgreen liquor. While it can be withdrawn through overflows in one of thetanks, it can also be withdrawn at a lower level, with the outflow thenbeing regulated by means of the pump 10. The latter case is to bepreferred.

FIG. 2 is a diagrammatic representation of a device according to theinvention as seen from above. The same reference numbers apply to thisfigure as to FIG. 1. The connecting line 3 between the tanks isexpediently designed in the manner shown in the figure so thatmechanical cleaning can be effected if, contrary to expectation, thepipes should become plugged.

Measuring equipment of a conventional type is present in each tank inorder to measure the concentration of the chemicals and the level of thesurface of the liquid. This measuring equipment expediently consists ofso-called bubble pipes and is not described in any more detail heresince these pipes are not included in the actual invention.

When the device according to the invention is to be used, smelt 5 isexpediently supplied in equal quantities to both the tanks. While thereis no objection per se to the two tanks being supplied with differentquantities of smelt, the supply of equal quantities is to be preferred.One of the tanks, for example tank II, is given a set point for thedesired concentration of the final solution, while the liquid in thesecond tank, for example I, is given a lower set point. The solution,the green liquor, is then only withdrawn from the system from tank II.Most of the weak liquor is added to tank I, which has the lower setpoint. The pump 10 for the outlet from this tank is closed at this stageand all the liquid which is added to tank I will therefore flow intotank II via the line 3. The level in tank I will therefore be somewhathigher than in tank II in proportion to the flow resistance in the line3. The quantities of weak liquor 6 which are to be added to thedifferent tanks can easily be calculated and set automatically. The setpoint in tank I is expediently half of the set point in tank II, whichmeans that the liquid which flows through the connecting line 3 is quiteweak and does not give rise to any deposition of pirssonite.

The addition of weak liquor, which has been shown diagrammatically at 6,should take place via the outlet pumps 10, which are then operating inthe opposite direction. This results in the outlet system being flushedand deposition being prevented.

In order to avoid the outlet systems becoming blocked, it is expedient,in accordance with the invention, for the functions of the tanks I andII to be switched over at regular intervals, for example once everytwenty-four hours or once every shift. The opening which was previouslythe outlet for the concentrated, final solution then becomes the inletopening for weak liquor. When alternating, the procedure is for the setpoint for the concentration in the tank from which pumping is to beginfirstly to be adjusted to the value which pumped-out green liquor is tohave. The flow of weak liquor to this tank will then decrease andperhaps cease for a period of time. When the desired concentration hasbeen reached, the appurtenant green liquor pump is started at the sametime as the other pump is stopped and the concentration set point of thecorresponding tank is adjusted down to a lower level. The influx of weakliquor will then become greater than is otherwise the case for a periodof time. The whole procedure can be automated and can be set in motionby an operation executed in the control room.

When the system according to the invention is used, the green liquorpumps provide complete backup for each other while, at the same time,the advantage is gained that the whole of each of the dissolver tanks,including the stirrer, is thoroughly washed at regular intervals with aweak green liquor, resulting in the dissolution of any deposits orencrustations. At the same time, the temperature will be altered, whichcan be advantageous for the cleaning process. Weak green liquor will, toall intents and purposes, always be flowing through the connecting pipebetween the tanks, so that the risk of deposition taking place in thispipe can be regarded as being negligible. Should such depositionnevertheless occur, contrary to expectation, early indication of thisfact will be obtained in the form of an increasing difference in levelbetween the tanks.

Another advantage of the invention is that regulation of theconcentration becomes more precise due to the step-wise addition of weakliquor.

EXAMPLE

The following is an example of running a plant according to theinvention:

A recovery boiler for 3200 t of dry substance per twenty-four hoursyields approximately 50 t of total titratable alkali per hour (50tTTA/h). If the green liquor from the dissolver tanks, prior to finaladjustment of the concentration, is assumed to be 175 kg of TTA/m³, andthe weak liquor contains 40 kg of TTA/m³, the total weak liquor flowbecomes 370 m³ /hour. The set point for the concentration in one of thetanks is adjusted to a value which corresponds to 120 kg of TTA/m³. Ifthe flow of smelt is divided equally between the two tanks,approximately 310 m³ of weak liquor will then enter one of the tanks perhour, and the remainder will enter the other tank. The flow in theconnecting pipe is also approximately 310 m³ /hour.

The invention is not limited to the embodiment which has been described,and can be varied in different ways within the scope of the patentclaims.

We claim:
 1. A process for dissolving smelt chemicals from recoveryboilers, the process comprising the steps of:providing a firstdissolving tank having a bottom portion, the first dissolving tankcontaining a first resulting solution having a first concentration ofsmelt chemicals; providing a second dissolving tank that is separatefrom the first tank, the second dissolving tank having a bottom portion,the second dissolving tank containing a second resulting solution havinga second concentration of smelt chemicals; providing supply devices forsupplying smelt chemicals into the first and second dissolving tanks;providing liquid supply members for supplying a dissolving liquid intothe first and second dissolving tanks so that the smelt chemicals andthe dissolving liquid form the resulting solutions disposed in the firstand second dissolving tanks, the supply of dissolving liquid and smeltchemicals being dependent upon the concentration of the smelt chemicalsin the resulting solutions; providing a first outlet defined in thefirst dissolving tank for permitting the first resulting solution toflow out of the first dissolving tank; providing a second outlet definedin the second dissolving tank for permitting the second resultingsolution to flow out of the second dissolving tank; providing aconnector connected to the bottom portions of the first and seconddissolving tanks so that the first dissolving tank is in fluidconnection with the second dissolving tank; providing a first shut offvalve in operative engagement with the first outlet, the first shut offvalve being movable between an open position that allows the firstresulting solution to flow through the valve and a closed position thatshuts off the flow of the first resulting solution; providing a secondshut off valve in operative engagement with the second outlet, thesecond shut off valve being movable between an open position that allowsthe second resulting solution to flow through the valve and a closedposition that shuts off the flow of the second resulting solution, thesecond shut off valve being independently operative relative to thefirst shut off valve; closing the first shut off valve and opening thesecond shut off valve; setting a first set point of the concentration ofthe smelt chemicals in the first dissolving tank; setting a second setpoint of the concentration of the smelt chemicals in the seconddissolving tank; conducting the first resulting solution to flow intothe second tank when the second set point is greater than the first setpoint; conducting the second resulting solution into the first tank whenthe first set point is greater than the second set point; and openingthe first shut off valve and closing the second shut off valve.
 2. Aprocess for dissolving smelt chemicals according to claim 1 wherein thefirst set point is different from the second set point and the steps ofsetting the first and second set points include the step of switching aset point value of the first set point with a set point value of thesecond set point every twenty-four hours.
 3. A process for dissolvingsmelt chemicals according to claim 1 wherein the steps of setting theset points include the step of setting the first set point twice as highas the second set point.
 4. A process for dissolving smelt chemicalsaccording to claim 1 wherein an approximately equal amount of smeltchemicals is supplied to the first and second dissolving tanks and amajority of the dissolving liquid is supplied to the first dissolvingtank if the set point of the second dissolving tank is greater than theset point of the first dissolving tank.
 5. A process for dissolvingsmelt chemicals according to claim 1 wherein the process furthercomprises the steps of changing the first set point of the firstdissolving tank relative to the second set point of the seconddissolving tank so that the flow of resulting solution through theconnector is alternated and raising the set point of the firstdissolving tank to a value that is the same as a value of the set pointof the second dissolving tank by decreasing the supply of dissolvingliquid to the first dissolving tank, opening the first shut off valveand closing the second shut off valve as the set point of the seconddissolving tank is lowered and the inflow of dissolving liquid to thesecond dissolving tank is increased.
 6. A process for dissolving smeltchemicals according to claim 1 wherein the step of alternating isperformed automatically.
 7. A process for dissolving smelt chemicalcomprising the steps of:providing a first and a second tank in fluidcommunication with one another, the first tank having a first outlet andthe second tank having a second outlet, the first outlet being closedand the second outlet being opened; providing the first tank with afirst concentration of green liquor and the second tank with a secondconcentration of green liquor, the second concentration being greaterthan the first concentration; conveying the green liquid in the firsttank into the second tank; opening the first outlet and closing thesecond outlet; increasing a supply of weak liquor into the second tankand reducing a supply of weak liquor into the first tank; conveyinggreen liquid in the second tank into the first tank; increasing thefirst concentration of green liquor of the first tank to the secondconcentration; and reducing the second concentration of green liquor ofthe second tank to the first concentration of green liquor.
 8. A devicefor dissolving smelt chemicals from recovery boilers, comprising:a firstdissolving tank having a bottom portion, the first dissolving tankcontaining a first amount of a first resulting solution having a firstconcentration of smelt chemicals; a second dissolving tank that isseparate from the first tank, the second dissolving tank having a bottomportion, the second dissolving tank containing a second amount of asecond resulting solution having a second concentration of smeltchemicals, the first amount being greater than the second amount, thesecond concentration being greater than the first concentration; supplydevices for supplying smelt chemicals into the first and seconddissolving tanks; liquid supply members for supplying a dissolvingliquid into the first and second dissolving tanks so that the smeltchemicals and the dissolving liquid form the resulting solutionsdisposed in the first and second dissolving tanks, the supply ofdissolving liquid and smelt chemicals being dependent upon theconcentrations of the smelt chemicals in the resulting solutions; afirst outlet defined in the first dissolving tank for permitting thefirst resulting solution to flow out of the first dissolving tank; asecond outlet defined in the second dissolving tank for permitting thesecond resulting solution to flow out of the second dissolving tank; aconnector connected to the bottom portions of the first and seconddissolving tanks so that the first dissolving tank is in fluidconnection with the second dissolving tank; a first shut off valve inoperative engagement with the first outlet, the first shut off valvebeing movable between an open position that allows the first resultingsolution to flow through the valve and a closed position that shuts offthe flow of the first resulting solution; and a second shut off valve inoperative engagement with the second outlet, the second shut off valvebeing movable between an open position that allows the second resultingsolution to flow through the valve and a closed position that shuts offthe flow of the second resulting solution, the second shut-off valvebeing independently operative relative to the first shut off valve.
 9. Adevice according to claim 8 wherein the device is a soda smeltdissolver.
 10. A device according to claim 8 wherein the liquid supplymembers comprises an adjustment valve having one end connected to acommon supply conduit and the other end connected to separate conduitsfor the first and second dissolving tanks.
 11. A device according toclaim 8 wherein each dissolving tank is provided with an outlet pumpthat is separately controllable.
 12. A device according to claim 8wherein the device comprises a bubble pipe measuring device formeasuring the concentration of smelt chemicals and a level of resultingsolution in the first and second dissolving tanks.